1. Field of the Invention
This invention pertains generally to the field of pyrometric devices and methods for temperature measurements, and more particularly, to pyrometers utilizing a narrow spectral band in the infrared region.
2. Description of the Background Art
Infrared pyrometry is an attractive temperature measurement technique. It is rapid and requires no physical contact with the object in which the temperature is being measured. Pyrometers operate by sensing the thermal radiation emitted by the heated object, and using the intensity of this detected radiation to determine the temperature of the object.
For a perfectly emitting and absorbing object (a "blackbody"), this determination is made by using the well-known "blackbody spectrum". This spectrum is given by the Planck radiation law, which describes the radiation intensity as a unique function of temperature and wavelength that is the same for all materials. However, in real objects the emitted thermal radiation depends also on the properties of the material surface. These properties are usually expressed as the emissivity of the material, which is the ratio of the radiation intensity actually emitted to that which would be emitted if the material were a perfect blackbody. This emissivity is a function also of the radiation wavelength and the material temperature. The value of the emissivity must be known in order to determine the temperature of the object from the intensity of the detected radiation. In the pyrometric method, the uncertainties in the emissivity are a fundamental limitation on the accuracy with which the temperature can be measured.
For example, infrared pyrometry is commonly employed in rapid thermal processors (RTP) to measure semiconductor wafer temperatures. The state of this technology has been reviewed in the article by L. Peters, "The Hottest Topic in RTP", published in Semiconductor International, Vol. 14, No. 9, August 1991, pp. 56-62. This article discusses various available RTP systems and pyrometric temperature monitors. The improvement of temperature control by minimizing the effect of emissivity variations is noted to be an important objective.
Narrow-band (narrow passband) pyrometers utilize a filtering technique to limit the range of radiation wavelengths measured by the system. In such systems the emissivity values are important only in the passband of the filter. It is desirable to choose the band for a range of wavelengths in which the emissivity is well known and constant. However, the emissivity generally varies with the temperature of the object and the surface characteristics.
In the case of silicon, the emissivity at wavelengths longer than the band edge is a strong function of the concentration of charge carriers. This concentration, in turn, depends on temperature. At low temperatures, which corresponds generally to low carrier concentration, silicon is nearly transparent. By increasing the temperature to approximately 500.degree. C., the emissivity of a silicon wafer of reasonable thickness becomes substantially constant at a value of about 0.7. Thus the accuracy of the normal pyrometric method in silicon at temperatures below 500.degree. C. is limited by the uncertainty in the emissivity values. The transparency of silicon at these temperatures also allows further errors arising from radiation from other sources that is transmitted through the wafer.
A further limitation in the pyrometric method is described in the article by D. W. Pettibone, J. R. Suarez and A. Gat, "The Effect of Thin Dielectric Films on the Accuracy of Pyrometric Temperature Measurement", Materials Research Society Symposium Proceedings, Vol. 52, pp. 209-216 (1988). These authors discuss the substantial effect of oxide and nitride films on the emissivity of silicon, and the resulting errors in pyrometric temperature measurements. In particular, the article reports the results of narrow-band pyrometric measurements at wavelengths around 3.5 microns, for silicon wafers having oxide films of various thicknesses. For temperatures between 650.degree. and 1100.degree. C., these measurement errors were of the order of 10.degree. to 50.degree. C. The authors were unable to make these measurements below 650.degree. C.